A method of the type described in the opening paragraph is known from the British patent publication GB 682 580. The object of the known method is to manufacture porous glass tubes, for example, for use as filters in laboratories. In which method it is important to obtain a highly uniform pore distribution and pore size between the sintered glass particles and to ensure that the pores are in communication with one another. For this purpose, suspensions of powdered glass having a highly uniform grain size are, prior to their introduction into the centrifuge where they form a loose bond of deposited solid particles on the inner surface, mixed under constant stirring with a binder which precludes settling of the solid particles. This procedure is followed to obtain a desired distribution of the solid particles in the suspension which is in conformity with the future porosity. A disadvantage of this method is that it requires a highly uniform grain size distribution of the starting material and, hence, correspondingly ground starting materials must be used. Moreover, grinding may lead to contamination of the material through abrasion. When commercially available powders are used, which have been formed through flame hydrolysis or precipitation, there is no longer a contamination risk, yet these powders generally exhibit a very coarse distribution of grain sizes. When such commerically available powders are used in the method described in GB 682 580, it is difficult to manufacture macroscopically, homogeneously built green bodies, since the particles of different particle size are separated in the centrifuging process and the deposited body then generally exhibits density gradients. During the further processing this may lead to problems such as, for example, cracks caused by contraction. Moreover, in the known method the filling of the centrifuge and the centrifuging process are operations which are separated in time, i.e. it is a discontinuous process, which renders the manufacturing process more expensive. Moreover, the formation of tubes having a greater wall thickness requires a repeated filling of the centrifuge, which involves time-consuming mounting operations. The process is particularly time-consuming when the porous body consists of many single layers, for example, having different compositions. In this case, the centrifuge has to be filled anew for each individual layer.
In a method known from the German patent application DE 3406148, powdered materials are used which are deposited in a thin liquid film by means of centrifugal force. In practice this method is satisfactory, but the feed rate of the powdered material must be adjusted to the rate of penetration in the liquid film. Otherwise there will be an accumulation of material on the liquid surface, which may lead to the formation of large agglomerates. In particular when commercially available powders are used these mechanisms limit the deposition rates and bring about inhomogeneities in the deposited solid body.